I consult with a lot of different startups... retail, restaurants, e-commerce... pretty much the whole shebang. And vitually all of them including yourself make the same mistake.
You are looking at this from entirely your own point of view. Wrong. Wrong. Wrong. Wrong.
If you want to find a product to sell or a business to start first climb into the shoes of the prospect. What problems are they facing and how can you solve them? The trouble with your dry-erase board concept is that you really aren't offering them anything unique or different that is going to solve their problems. Thusly they aren't going to give their hard-earned scratch to fund it.
What can you do to really HELP a business drive more qualified (the key word being "qualified") traffic into their places of business? A dry-erase board with a businss card sized ad or some other poorly copywritten (not that you would necessarily do it poorly but I'm seen too damn many that are) isn't going to drive qualified traffic. And most business owners are smart enough to understand that. Give them something that really works.
You want to start a business? Find a group of underserved prospects that aren't having their needs met by their current suppliers. You step in and fill the need and thusly they fill your pockets with gold.
You want a good example? Here's one...
Smith & Hawken, the tool people recognized that simple garden hand type tools were the red-headed step children of most garden centers. They were there strictly as an add-on sale. Most of the time these tools were nothing more than cheap-ass imports that bent or broke after a few casual uses. Smith & Hawken realized there were some damn good gardeners tools out there, many coming from a company in England that even offered a life-time warranty on their products. Smith & Hawken imported these products and started selling them through direct response ads and soon after a catalog. They (S&H) educated consumers on "why" these top of the line tools were a good investment... in other words they put it in dollars and sense the consumer could understand and appreciate. Guess what sales took off like gang-busters! S&H realized that most garden hand tools were crap and offered an alternative that has made them a ton of money. This isn't rocket science. It's finding underserved niches in the market and exploiting them for all they're worth.
And you don't have to start with a whole lineup of products (or services) either start with one thing. One thing that can solve a problem or fills a need. Once that one product or service has proven itself to be profitable then expand it like crazy!
I don't see where your dry-erase board solves a problem or fills a need that is superior to all their (the business owners) other options. You do not have a USP (unique selling proposition).
Take care,
Mike Winicki-Business Specialist and Strategic Planner